Gas filtration is critical in semiconductor manufacturing environments. Tremendous efforts are made to eliminate yield-reducing contaminants from the gases used in semiconductor processing tools. Contaminants can generally be classified as either particulate or molecular. Common particulate contaminants include dust, lint, dead skin, and manufacturing debris. Examples of yield-reducing contaminants include: acids, such as hydrobromic acid, nitric acid, sulfuric acid, phosphoric acid, hydrochloric acid; bases, such as ammonia, ammonium hydroxide, tetramethylammonium hydroxide, trimethylamine, triethylamine, hexamethyldisilazane, NMP, cyclohexylamine, diethylaminoethanol, methylamine, dimethylantine, ethanolamine, morpholine, condensables such as silicones and hydrocarbons with a boiling point greater than or equal to 150° C.; and dopants such as boron (usually as boric acid), phosphorous (usually as organophosphate), and arsenic (usually as an arsenate).
In semiconductor photolithography tools gas is supplied for generally two purposes: the actuation of tool pneumatics; and the purging of tool optics. Although purified dry air, nitrogen, or the like is generally used to drive pneumatics and purge optics, small amounts of contaminants are still liable to be present in the gas at concentrations sufficient to damage tool optics (for example, illuminator optics and projection lenses). Contaminating substances may adhere onto the optical elements to form molecular films. Molecular films on optical surfaces physically absorb and scatter incoming light. Scattered or absorbed light in photolithography optical surfaces causes distortion of the spherical quality of wavefronts. When the information contained in the spherical wavefront is distorted, the resulting image is also misformed or abberated. Image distortions, or in the case of photolithography, the inability to accurately reproduce the circuit pattern on the reticle, cause a loss of critical dimension control and process yield.
Contaminating substances may also chemically react with the optical surfaces of the photolithography tool and/or the wafers being processed in the tool. For example, sulfur dioxide may combine with water in the tool to produce sulfuric acid, which can irreversibly damage tool optics. In addition, ammonia may react with wafer surface materials, such as the resist, gate-insulating films, and the like, interfering with the photolithography processing step and reducing process yield. Thus, the purity of the gases supplied to semiconductor processing tools is of critical concern.